Monolithic implementation of adjustable linear passive components employing conventional fabrication methods has been impractical if not unrealizable due to the difficulty in fabrication and expense of fabrication of these components on an integrated circuit. Recently, the problems associated with the fabrication of these devices have been addressed by employing MEMS technology. MEMS technology is a process for fabricating various components using micromachining in a similar manner to fabricating integrated circuits (ICs). MEMS structures are typically capable of mechanical motion or force and can be integrated onto the same device structure with electronic devices that provide the stimulus and control of the mechanical structures. Many different variety of MEMS devices (e.g., microsensors, microgears, micromotors) have been fabricated employing MEMS technology. Additionally, variable passive devices (e.g., inductors, capacitors) can be fabricated employing MEMS technology as micron-sized electromechanical structures.
Electrostatic forces are employed to move structures by energizing one or more electrodes coupled to a movable structure and one or more electrodes coupled to a base structure. Electrically energizing the electrodes creates an electrostatic force that attracts the electrodes to one another, usually against a spring restoring force. A typical MEMS electrostatically variable capacitor includes two parallel plates in which a fixed plate is provided on a substrate and a movable plate is disposed above the fixed plate and is movable toward and away from the fixed plate. The distance between the two plates is variable and thus, determines the capacitance of the capacitor. Both plates are coupled to electrodes to generate the electrostatic forces that move the movable plate toward the fixed plate, balancing against a spring restoring force. A signal line is also coupled to the movable plate and the fixed plate which provides the electrical signal to the capacitor. The tuning range of the variable capacitor is limited by the distance over which the movable plate can be controlled. The change in distance between the movable plate and the fixed plate that can be achieved limits the dynamic range of the variable capacitor in addition to the capacitive values.
Attempts to provide variable inductors have been made employing MEMS structures. For example, the inductance of an inductor coil may be varied by moving a magnetic material axially into and out of the inductor coil. However, magnetic materials are not easily implemented in a MEMS device since most materials available have poor material permeability in addition to experiencing losses at high frequencies. Another mechanism for providing a variable inductor is to dispose a first coil within a second coil connected electrically in parallel and vary the inductance of the second coil by rotating the first coil on an axis disposed in the plane of the second coil. A rotatable motor or the like is necessary to rotate the first coil, which is complicated to implement in a MEMS device.